In multi-color printing, colors are laid down on a moving web by separate printing units which print in different colors such as cyan, yellow, magenta, and black. The images printed by the separate printing units must be printed one on top of the other. This is known as color registration.
Closed loop control of color registration has been previously proposed. See, for example, U.S. Pat. No. 4,546,700, issued to Kishner et al. and incorporated herein by reference. Kishner et al. discloses a method and apparatus for automatically sensing and maintaining registration of each of the color cylinders in a multi-color printing press. Predetermined register marks are printed in non-image areas on the sheet to be printed. A register mark for each process color is printed in the same location as a corresponding black reference register mark. The reference register marks are configured such that the average reflectance of the printed overlay will change as the registration of the color image changes with respect to the black image. The reflectance of each printed overlay is measured on the printing press, while the sheet is moving. One type of sensor that can be used for this measurement is an on-press densitometer. If desired, the reflectance information obtained thereby can be converted to a registration error signal and used to achieve automatic, closed-loop control of the position of each of the color cylinders.
See also, for example, U.S. Pat. No. 4,534,288, issued to Brovman and incorporated herein by reference. Brovman proposes a unified system for detecting misregistration and for ink density control in a four color printing press. A color bar having 136 square fields is printed in a non-image area normally trimmed or otherwise removed from the finished product. The color bar includes register fields. The percentage of non print area in register indicia is detected and used to control register adjustment mechanisms of the press.
As evidenced by the Kishner et al. and Brovman patents, closed loop color registration systems typically require marks to be printed outside the image areas of a sheet, usually in a margin. In addition, closed loop color registration systems typically require a separate registration mark, spaced from the other registration marks, for each color.